The authoritative bullshit isn't what society is running on, it's what society is giving as an excuse for enshittification that enriches interested parties.
Your landlord (I'm guessing based on having seen it before) gets kickbacks from the ISP to force all tenants onto a specific (probably overpriced) Internet plan. The interest in keeping you from configuring your own router is in allowing the ISP's enshittifying further monetization tactics to proceed unopposed. The two big ones I've seen in this kind of setup are:
Using DNS enforced by the router to gather data and place ads on any 404 error.
Sharing their WiFi network that you lease with the ISP's other customers nearby.
I'm not even confident they get kickbacks. They probably just believe that either negotiating service details or providing infrastructure for competitive options would require more work from them. They are probably right about that: I'm not hassling them over it. I'm really not in a good position to anyway. As much as I would like to attribute malice to this behavior, it's most likely to be no more than laziness.
If I moved into a house, I could get 1gbit symmetric from Google Fiber or UTOPIA at half the price. But that doesn't matter because I cannot remotely afford a mortgage.
The real problem is Monopoly. Not the market dominance kind: the no one gets to compete kind. We have it in real estate, where every piece of the market is overvalued so far that very few individual people can meaningfully participate. We have it with ISPs who get to literally own the last mile infrastructure, so their customers can't physically connect to a competitor.
Your landlord (I'm guessing based on having seen it before) gets kickbacks from the ISP to force all tenants onto a specific (probably overpriced) Internet plan. The interest in keeping you from configuring your own router is in allowing the ISP's enshittifying further monetization tactics to proceed unopposed. The two big ones I've seen in this kind of setup are:
Using DNS enforced by the router to gather data and place ads on any 404 error.
Sharing their WiFi network that you lease with the ISP's other customers nearby.